why does electron revolve around the nucleus?

The electrons are particles which contain a unit negative charge in them. The protons are the ones which contain unit positive charge in them. When they come enough close to each other there exists attractive force between them.

The electron has some velocity with which it moves when it gets pulled by a proton, the velocity of electron tries to move it in the direction in which it was moving and proton tries to attract it wholly in to it. So, there is some centripetal force by proton on electron and some centripetal acceleration on it. But, the previous velocity of electron doesn’t let the proton attract it wholly. So, the electron has tangential velocity as well as some centripetal acceleration on it. And therefore, the resultant velocity acts and aelectron follows that circular path.